1. Field of the Invention
The present invention relates to a driver for applying drive power to a movable body by making use of the expansion and contraction of an electromechanical energy transducer, and, more particularly, to a driver which can be reduced in size by using fewer parts and which generates a large drive power.
2. Description of the Related Art
FIGS. 1 to 7 in Japanese Unexamined Patent Application Publication No. 7-308083 (Patent Document 1) illustrates a driver having a structure in which a movable portion 2 is inserted in an inner peripheral surface (guide surface 1a) of a cylindrical guide 1. In the driver, the movable portion 2 is joined to a driving portion 3 for driving the movable portion 2 in an axial direction of the guide 1, and an inertial portion 4 is joined to the driving portion 3. The movable portion 2, the driving portion 3, and the inertial portion 4 are inserted in the guide surface 1a. 
A presser member 6 having the form of a tongue piece is formed at the movable portion 2.
The driving portion 3 has a piezoelectric element 7 which expands and contracts in an axial direction of the guide 1 for generating drive power at the movable portion 2. The inertial portion 4 is joined to the piezoelectric element 7.
The inertial portion 4 includes a disc-shaped holding plate 9 and a lens 10 held by and secured to the holding plate 9.
In the driver disclosed in Patent Document 1, a side edge 6e of the presser member 6 contacts the guide surface 1a while the side edge 6e is exhibiting resiliency. Frictional force generated by the contact causes the movable portion 2 and the driving portion 3 to be supported by the guide surface 1a. 
In the driver, as illustrated in FIG. 3 in Patent Document 1, when the piezoelectric element 7 is expanded rapidly by rapidly applying a voltage to the piezoelectric element 7, inertial force of the inertial portion 4 overcomes the frictional force between the presser member 6 at the movable portion 2 and the guide surface 1a, so that the movable portion 2 moves in the direction of arrow Ra shown in FIG. 3A in Patent Document 1.
After the movement of the movable portion 2, if the voltage being applied to the piezoelectric element 7 is gradually reduced, the piezoelectric element 7 gradually contracts. Here, the movable portion 2 remains at rest by the frictional force between the guide surface 1a and the presser member 6 at the movable portion 2.
After gradually expanding and contracting the piezoelectric element 7 by gradually applying the voltage to the piezoelectric element 7, if the voltage being applied to the piezoelectric element 7 is rapidly reduced, as shown in FIG. 5 in Patent Document 1, the movable portion 2 can be moved in a direction opposite to that shown in FIG. 3 in Patent Document 1.
A driver making use of the same principle and having a similar structure is illustrated and disclosed in FIGS. 8 to 13 in Patent Document 1.
By repeating the expansion and contraction of the piezoelectric element 7 as described above, the movable portion 2 is moved in an axial direction of the guide 1.
In the driver disclosed in Patent Document 1, the frictional force for supporting the movable portion 2 at the guide surface 1a is produced by the resilient presser member 6. Therefore, in the driver disclosed in Patent Document 1, the presser member 6 is indispensable to properly moving the movable portion 2. Consequently, the number of parts is increased, as a result of which there is a limit as to how small the driver can be made.
In the driver disclosed in Patent Document 1, the piezoelectric element 7 for generating drive power is formed at the driving portion 3 inserted in the guide surface 1a of the guide 1. Here, the guide surface 1a is formed by an inner peripheral surface of the cylindrical guide 1. Therefore, there is a limit as to how large it can be made.
Therefore, parts, such as the presser member 6, must be formed in a small space, thereby making it difficult to manufacture the parts and making it necessary to make the piezoelectric element 7 small. Consequently, there is a limit as to how large the drive power that is generated can be.